perf test: Check for linking problems in the python binding

It just will add the O= builddir to PYTHONPATH and try to 'use perf',
which will, in verbose mode show the python backtrace with the missing
symbols, such as in the problem fixed in the patch after this one:

  # perf test -v 15
  15: Try 'use perf' in python, checking link problems       :
  --- start ---
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  ImportError: /home/acme/git/build/perf//python/perf.so: undefined symbol: test_attr__enabled
  ---- end ----
  Try 'use perf' in python, checking link problems: FAILED!
  #

Loooong overdue, done.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-zmd2oq9gz6t1u145ub7qm2nv@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>